1. Field of the Invention
The present invention relates to a zoom lens system, which has a half angle-of-view of 35 degrees or more covering a wide-angle area, is ideal as an imaging optical system mainly for an image sensor.
2. Description of Related Art
In recent years, the demands of zoom lens systems including a wide angle-of-view have risen in compact digital cameras. Whereas, miniaturization of the pixel-pitch has progressed, and there are demands for the optical system to have a high resolution. Mainly in imaging lens systems for compact cameras, a slim camera design is often achieved by employing a retractable (telescoping) mechanism which reduces the air-distance between each lens group at the fully-retracted position (accommodated state). In the imaging lens systems which are suitable for such retractable mechanisms, in addition to the need for the thickness of each lens group to be smaller (thinner), at the design stage, it is necessary to consider the structural restrictions on the overall length of the optical system as well as on the frontmost lens diameter which is to be smaller.
Zoom lens systems for high zoom-ratio compact cameras are known in the art, e.g., Japanese Unexamined Patent Publication Nos. 2006-23529 and 2006-308957, in which the number of lens elements is relatively few, and miniaturization is achieved.
However, in the zoom lens system of Japanese Unexamined Patent Publication No. 2006-23529, the zoom ratio is approximately 5:1, and the angle-of-view at the short focal length extremity is 61 degrees; hence, a sufficient wide angle-of-view and a high zoom ratio are not achieved.
In the zoom lens system disclosed in Japanese Unexamined Patent Publication No. 2006-308957, a wide angle-of-view of approximately 78 degrees is achieved; however, the zoom ratio is approximately 4:1 through 7:1, which is unsatisfactory.
The zoom lens system disclosed in Japanese Unexamined Patent Publication No. 2008-112013 achieves a relatively compact zoom lens system having a wide angle-of-view and a high zoom ratio; however, large amounts of astigmatism and chromatic aberration occur, which is unsatisfactory for a zoom lens system that is required to attain high pixelization.